1. Field of the Invention
The present invention relates generally to optical grating devices, and more particularly to techniques for athermalizing such devices.
2. Description of Related Art
Computer and communication systems place an ever-increasing demand upon communication link bandwidths. It is generally known that optical fibers offer a much higher bandwidth than conventional coaxial links. Further, a single optical channel in a fiber waveguide uses a small fraction of the available bandwidth of the fiber. In wavelength division multiplexed (WDM) optical communication systems, multiple optical wavelength carriers transmit independent communication channels at different wavelengths into one fiber. By transmitting several channels at different wavelengths into one fiber, the bandwidth capability of an optical fiber is efficiently utilized.
Fiber-optic multiplexing and demultiplexing have been accomplished using an arrayed waveguide grating (AWG) device. An AWG is a planar structure comprising an array of waveguides disposed between input and output couplers and arranged side-by-side with each other, and which together act like a diffraction grating in a spectrometer. Each of the waveguides differs in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the output coupler form the outputs of the multiplexing and demultiplexing device. In operation, when a plurality of separate and distinct wavelengths are applied to separate and distinct input ports of the device, they are combined and are transmitted to an output port. The same device may also perform a demultiplexing function in which a plurality of input wavelengths on one input port of the apparatus, are separated from each other and directed to predetermined different ones of the output ports. AWGs can also perform a routing function, in which signals arrive on multiple input ports and are routed to multiple different output ports in accordance with a predefined mapping. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 2 Jun. 1996, and U.S. Pat. No. 5,002,350 and WO97/23969, all incorporated by reference.
Wavelength division multiplexers and demultiplexers require precise control of the effective optical path length difference between adjacent waveguides. The effective optical path length difference is defined as the product of the effective index of refraction of the fundamental mode in the waveguide and the physical path length difference between adjacent waveguides. The effective index of refraction of the fundamental mode in the waveguides and the physical path length differences between adjacent waveguides for currently available wavelength division multiplexers and demultiplexers are typically both temperature dependent. In conventional integrated optical multiplexer and demultiplexer devices, the medium forming the arrayed waveguides has a noticeable temperature dependency which results in changes in the central transmission wavelength which may exceed the transmission bandwidth. As a result, temperature variations that are within a specified device operating temperature range (e.g. from about 0 C to about 70 C) induce a wavelength shift which is unacceptable in comparison to the typical accuracy requirements. Consequently, available multiplexer/demultiplexer optical devices of the phased array type are generally operated in a temperature controlled environment. Typically, control circuits with heating elements are provided to maintain the device at a stable temperature higher than the maximum specified operating temperature. But the use of heating elements to achieve active athermalization is undesirable because it increases the overall cost, size and complexity of the device, reduces device lifetimes, and consumes considerable power. It also usually requires active smart control electronics and even then it may operate differently depending on the device's physical horizontal/vertical orientation. Peltier coolers can also be used, but these suffer from many of the same inadequacies.
In the case of conventional wavelength division multiplexers having a phased array optical grating comprising a plurality of silica core waveguides and silica cladding, the variation of channel wavelength as a function of temperature predominately depends on the positive variation of the effective index of refraction of the silica as a function of temperature. In an effort to compensate for the positive variation of refractive index as a function of temperature for silica-based materials, polymer overcladding materials having a negative variation of refractive index as a function of temperature have been employed. However, a problem with this arrangement is that as the temperature varies, the difference in refractive index between the core and the cladding varies, and in the worst case, light may not be able to be guided into the waveguide. As a result, optical multiplexer/demultiplexer devices having a phased array type grating with a polymer overcladding may not be suitable for use over a wide range of ambient temperatures.
Another proposed design for maintaining a relatively constant effective optical path length difference between adjacent waveguides in a phased array involves localizing a compensation material (typically a polymer) in a triangular or crescent-shaped groove or slot either in the phased array or in the slab region coupling the phased array with either the input or output fibers. The polymer-filled groove is transversely oriented across adjacent waveguides in a direction generally perpendicular to the direction of the optical paths. The physical length of the groove along the optical path of a given waveguide is a function of the physical path length, such that the optical paths of adjacent waveguides experience a constant physical path length increment through the groove. The polymer can be selected such that it has a negative variation in effective index of refraction as a function of temperature to compensate for the positive variation in the index of refraction of the silica waveguide core segments as a function of temperature, thereby inhibiting shifting of channel wavelengths due to variations in operating temperature within a predetermined operating temperature range. The polymer groove can be divided into more than one groove encountered by the optical energy sequentially, to reduce the length of free space propagation across each groove.
The use of polymer-filled grooves can improve athermalization substantially. In conventional polymer-filled groove athermalization methods, the change in the respective index of refraction of each of the silica based waveguide and the polymer compensation material, are both assumed to be linear with temperature. Any higher order effects typically are ignored. Typical AWGs that have been athermalized in this way can achieve a center channel wavelength drift as small as 0.03 to 0.05 nm over a typical operating temperature range of +5 to +70 C. However, that is still not good enough for many applications. Such drifts limit the applicability of the device to only that stated temperature range, and to only systems having channel spacings of about 100 GHz or higher, where this variation would be tolerable. They are not readily usable, for example, in an outdoor equipment enclosure in climates where freezing temperatures are possible, or in systems that require a broad passband and a channel spacing less than about 100 GHz.
Another major category of techniques that have been investigated for athermalization are mechanical in nature, such as techniques that include temperature-controlled actuators for actively positioning the components of the device relative to each other. These may include, for example, a bi-metallic actuator that adjusts the lateral position of the input waveguide relative to the input slab region in accordance with ambient temperature. These techniques are generally complex and expensive to make as the manufacturing tolerances are usually extremely tight.
Accordingly, it is desirable to provide arrayed waveguide grating devices that exhibit excellent athermalization over a wide temperature range, without requiring a temperature controlled environment, and without requiring the complexities and tight manufacturing tolerances of mechanical methods.